The invention relates to fluidised-bed combustion apparatus.
In this specification "combustion apparatus" includes boilers for supplying hot water or for heating other fluids, boilers for producing steam and apparatus for producing hot gas or for incineration.
Fluidised-bed apparatus such as shell boilers have typically for example a shell in which a horizontal furnace tube extends from a containment for a fluidisable bed to a combustion chamber.
In such boilers particles of bed material and ash are elutriated from the bed and are conveyed by the gas leaving the bed for varying distances downstream of the bed. Some elutriant falls out of the gas stream and is deposited in the furnace tube. Some is conveyed right through the furnace tube and is deposited in the combustion chamber, for example. Some is deposited in the smoke-tubes and finer particles are conveyed to the rear smoke-box and even out through the smokestack. Generally, a grit-arrestor is provided to reduce the emission of fine particles from the stack as far as possible.
The rate of elutriation of material from the fluidised bed is dependent on the rate of firing of the boiler and upon the rate of flow of gas through the bed. The degree to which elutriant is deposited at any given point is dependent upon the rate of flow of gas, which in turn depends upon the rate of flow of gas through the bed and the rate of firing. The gas flow leaving the bed is not uniform, the rate of flow varying from point to point across the furnace tube.
For example, in a shell boiler rates at 10,000 lbs of steam/hour (4545 kg of steam/hour) which had a bed of alumina (having an initial bed weight of approximately 400 kg, a mean particle size of 700 um and a particle terminal velocity of 9.6 m/s at 950.degree. C.), it was found that the combustion/fluidisation gasses (which has a velocity of approximately 32 m/s over a rear wall of the bed containment) elutriated approximately 112 kg/hour of bed material. Of that total of elutriated material, approximately 100 kg was deposited in the furnace tube and the combustion chamber and approximately 12 kg was deposited in the rear smoke-box.